Brake problem on 1990 Chevrolet Camaro

Rear brakes will not come on when pedal is moved. This is a 1979 Camaro. We have tried master cylinder, proportional valve and checked the line for restrictions. Front brakes are fine, but rear will not engage with or without the engine running.

Thanks. I finally figured out the problem. We were bleeding the brakes with the old pedal method and were not getting any rear brakes. We power bled the system and now the brakes work properly. There must be something in the system that requires pressure to re-set the system. Again, thanks for your help.